The present invention relates to a hydrogenated vinyl aromatic hydrocarbon resin composition having excellent transparency, heat resistance and adhesiveness to metals, etc., and an optical disc substrate obtainable by using such a composition.
Extensive studies have been made recently for development of optical recording utilizing laser as such optical recording enables high density recording of information, its storage and reproduction. A typical example of the devices embodying the principle of such optical recording is optical disc. Generally, optical disc comprises basically a transparent substrate and various recording media coated thereon.
Colorless and transparent synthetic resins are favorably used for the transparent substrate of optical disc, and preferred examples of such synthetic resins are polycarbonate (hereinafter abbreviated as PC), polymethyl methacrylate (hereinafter abbreviated as PMMA) and polystyrene (hereinafter abbreviated as PSt). These resins are not only prominent in colorless transparency but also possess their own excellent properties. However, these resins do not satisfy with all the requirements for application to optical material, especially optical disc substrate, and they still have the problems to be solved. For example, PC is defective in birefringence properties due to its aromatic ring. It is also high in water absorptivity and water permeability. In PMMA, the problems relating to heat resistance, water absorptivity and toughness have been pointed out. In PSt, although satisfactory in water absorptivity and water permeability, it causes very wide birefringent index and is also poor in heat resistance.
Thus, these resins are practically used with their own technical defects left outstanding. Actually, there are noted the new technical defects such as mentioned later in relation to the recording media coated on the transparent substrates made of the said resins.
Regarding the recording media, there have been made wide-ranging researches and various types of recording media have been developed in accordance with the use of optical disc. For example, there is known a perforated-type recording media used exclusively for recording and reproduction, which are called "write once"-type recording media. Also, in the "erasable"-type recording media which are capable of recording, reproduction, erasion and re-recording, there are known a phase transition-type recording media utilizing the phenomenon of crystal phase transition and a photomagnetic-type recording media utilizing the photomagnetic effect. As for the materials for these magnetic media, there are mostly used tellurium, tellurium oxides and tellurium-based alloy compounds for the "write once"-type recording media, and amorphous alloy compounds of rare earth and transition metals such as GdFe, TbFe, GdFeCo and TbFeCo, and inorganic materials for the "erasable"-type magnetic media. Generally, these magnetic media are made by forming a film of the said material on a transparent substrate by a dry method such as sputtering under high vacuum.
High hygroscopicity and water permeability of PC and PMMA cause the technical defects. That is, high hygroscopicity of the said materials gives rise to the problem of warp of the substrate due to its expansion on moisture absorption. Also, high water permeability of the said materials gives rise to the problem that corrosion of the magnetic medium is caused by water which gets into the medium through the substrate, resulting in a reduced service life of the optical disc.
With reference to heat resistance of the resins for substrate, there exist the following technical problems. In practical use of optical discs, especially "write once"-type ones and "erasable"-type ones, the temperature of the recording medium at the time of writing of record or erasion may reach 200.degree. C. or higher. It is therefore liable that the disc substrate will be heated to a fairly high temperature at the time of writing of record or erasion, although the heat of the recording medium may not be directly applied to the substrate, and as a consequence there may arise the problem of deformation of the substrate or the grooves in case where the resin used is low in heat resistance.
In the production process of optical discs, there is often incorporated a heat treatment step for the purpose of preventing change with the passage of time of the substrate or magnetic medium. However, for the improvement of productivity, it is desirable that the heat treatment be conducted at as high a temperature as possible to minimize the treatment time.
Use of a resin with low heat resistance makes it impossible to adapt a high treating temperature, making it unable to raise productivity.
For these reasons, PSt and PMMA which are low in heat resistance, are quite unsatisfactory for enduring high temperature in the production process of optical discs or in use thereof, and PC having high heat resistance has exclusively been studied as an available material for transparent substrate.
Use of a transparent resin with high softening point, which is principally composed of carbon and hydrogen, is known as a resin for making up for the defects of the conventional resins such as PC, PMMA and PSt. A hydrogenated vinyl aromatic hydrocarbon polymer is proposed in Japanese Patent Application Laid-Open (KOKAI) No. 318015/89.
However, this hydrogenated vinyl aromatic hydrocarbon polymer, being no exception to the inherent low adhesiveness of polyolefins, has a technical defect that it was poor in adhesion to the recording film and incapable of satisfying the requirement for long life of optical disc.
As a result of strenuous studies for improving its adhesiveness to the recording layer, it has been found that by using as a raw material of the substrate, a hydrogenated vinyl aromatic hydrocarbon polymer composition obtainable by blending a hydrogenated vinyl aromatic hydrocarbon polymer having specified hydrogenation degree and a hydrogenated vinyl aromatic hydrocarbon-conjugated diene block copolymer having specified composition ratio and hydrogenation degree an optical disc substrate having excellent adhesiveness to the recording layer while maintaining high heat resistance in which the hydrogenated vinyl aromatic hydrocarbon polymers inherently posses, can be obtained. The present invention was attained on the basis of this finding.